Generate Mesh Using Boolean Modifier Over Time
Array with end caps

Example using default cylinder and UV sphere. (matching loop vert count)

Add a UV sphere in edit mode select half and P separate by selection.



Add the default cylinder with no ends (None) Add an array modifier.  Use the two sphere halves as end caps.  Make the relative offset in z (arbitrary, chosen since it is  default alignment)



We now have our capsule shape... but not the ball.

Add a shapekey to the cylinder, and make this zero scale in Z. SZ0  Now whenever the influence of this shapekey is 1, the whole thing will be a ball, no matter what the count is on the array modifier.



Put together quickly from default primitives as a proof of concept.  Once the shapekey is &lt; 1 the length of trail can be adjusted using array modifier, as well as shapekey, depends on the curliness of the path.



Re making a motion trail

Could script this to make a motion path based on the transform of the object.. Could be tricky re handling bounces etc.